26 Search Results

We present the result of a search for the charged-lepton-flavor violating decays τ⁻ → e^∓ℓ^±ℓ⁻, where ℓ is a muon or an electron, using a data sample with an integrated luminosity of 428 fb⁻¹ recorded by the Belle II experiment at the SuperKEKB e⁺e⁻ collider. The selection of e⁺e⁻ → τ⁺τ⁻ events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background. Upper limits on the branching fractions between 1.3 and 2.5 × 10⁻⁸ are set at the 90% confidence level. These results are the most stringent bounds to date formore » four of the modes.« less

We report a search for an axion-like particle a in B → K^(*)a decays using data collected with the Belle detector at the KEKB asymmetric-energy electron-positron collider. The search is based on a 711 fb⁻¹ data sample collected at the Υ(4S) resonance energy, corresponding to a sample of 772 × 10⁶ Υ(4S) events. In this study, we search for the decay of the axion-like particle into a pair of photons, a → γγ. We scan the two-photon invariant mass in the range 0.16 GeV–4.50 GeV for the K modes and 0.16 GeV–4.20 GeV for the K^* modes. No significant signalmore » is observed in any of the modes, and 90% confidence level upper limits are established on the coupling to the W boson, g_aW, as a function of a mass. The limits range from 3 × 10⁻⁶ GeV⁻¹ to 3 × 10⁻⁵ GeV⁻¹, improving the current constraints on g_aW by a factor of two over the most stringent previous experimental results.« less

We recently measured the branching fraction of the 𝐵⁺→ 𝐾⁺⁢$$𝑣\bar{𝑣}$$ decay using 362 fb⁻¹ of on-resonance 𝑒⁺⁢𝑒⁻ collision data under the assumption of Standard Model kinematics, providing the first evidence for this decay. To facilitate future reinterpretations and maximize the scientific impact of this measurement, we publicly release the full analysis likelihood along with all necessary material required for reinterpretation under arbitrary theoretical models sensitive to this measurement. In this work, we demonstrate how the measurement can be reinterpreted within the framework of the weak effective theory. Using a kinematic reweighting technique in combination with the published likelihood, we derivemore » marginal posterior distributions for the Wilson coefficients, construct credible intervals, and assess the goodness of fit to the Belle II data. For the weak effective theory Wilson coefficients, the posterior mode of the magnitudes |𝐶_VL+𝐶_VR|, |𝐶_SL +𝐶_SR|, and |𝐶_TL| corresponds to the point (11.3, 0.0, 8.2). The respective 95% credible intervals are [1.9, 16.2], [0.0, 15.4], and [0.0, 11.2].« less

The Belle II experiment’s ability to identify particles critically affects the sensitivity of its measurements. We describe Belle II’s algorithms for identifying charged particles and evaluate their performance in separating pions, kaons, and protons using $$426\,\text {fb}^{-1}$$ of data collected at the energy-asymmetric $$e^{+}e^{-}$$ collider SuperKEKB in 2019–2022 at center-of-mass energies at and near the mass of the $$\Upsilon$$ (4S).

We present a study of the rare charm meson decays 𝐷⁰ →𝐾⁺⁢𝐾⁻⁢𝑒⁺⁢𝑒⁻, 𝜋⁺⁢𝜋⁻⁢𝑒⁺⁢𝑒⁻, and 𝐾⁻⁢𝜋⁺⁢𝑒⁺⁢𝑒⁻ using a 942  fb⁻¹ dataset collected by the Belle detector at the KEKB asymmetric-energy 𝑒⁺⁢𝑒⁻ collider. We identify 𝐷⁰ candidates via the charge of the pion from 𝐷*⁺ →𝐷⁰⁢𝜋⁺ decays and normalize the branching fractions to 𝐷⁰ → 𝐾⁻⁢𝜋⁺⁢𝜋⁻⁢𝜋⁺ decays. The branching fraction for decay 𝐷⁰ → 𝐾⁻⁢𝜋⁺⁢𝑒⁺⁢𝑒⁻ is measured to be (39.6 ± 4.5 (stat) ± 2.9 (syst)) × 10⁻⁷, with the dielectron mass in the 𝜌/𝜔 mass region 675 < 𝑚_𝑒⁢𝑒 < 875  MeV/⁢𝑐². We also search for 𝐷⁰ → ℎ⁻⁡ℎ^(′)+⁡𝑒⁺⁢𝑒⁻ (ℎ^(′) = 𝐾, 𝜋) decays withmore » the dielectron mass near the 𝜂 and 𝜙 resonances, and away from these resonances for the 𝐾⁺⁢𝐾⁻⁢𝑒⁺⁢𝑒⁻ and 𝜋⁺⁢𝜋⁻⁢𝑒⁺⁢𝑒⁻ modes. For these modes, we find no significant signals and set 90% confidence level upper limits on their branching fractions at the 𝒪⁡(10⁻⁷) level.« less

We present a search for the rare flavor-changing neutral-current decay 𝐵⁰→𝐾^*0⁢𝜏⁺⁢𝜏⁻ with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb⁻¹ data sample recorded at the center-of-mass energy of the ϒ⁡(4⁢𝑆) resonance. One of the 𝐵 mesons produced in the ϒ⁡(4⁢𝑆)→𝐵⁰$$⁢\overline{𝐵}$$⁰ process is fully reconstructed in a hadronic decay mode, while its companion 𝐵 meson is required to decay into a 𝐾^*0 and two 𝜏 leptons of opposite charge. The 𝜏 leptons are reconstructed in final states with a single electron, muon, charged pion or charged 𝜌 meson, and additional neutrinos.more » We set an upper limit on the branching fraction of ℬ⁡(𝐵⁰→𝐾^*0⁢𝜏⁺⁢𝜏⁻)<1.8×10⁻³ at the 90% confidence level, which is the most stringent constraint reported to date.« less

Inelastic dark matter models that have two dark matter particles and a massive dark photon can reproduce the observed relic dark matter density without violating cosmological limits. The mass splitting between the two dark matter particles 𝜒₁ and 𝜒₂, with 𝑚⁡(𝜒₂) >𝑚⁡(𝜒₁), is induced by a dark Higgs field and a corresponding dark Higgs boson ℎ′. We present a search for dark matter in events with two vertices, at least one of which must be displaced from the interaction region, and missing energy. Using a 365 fb⁻¹ data sample collected at Belle II, which operates at the SuperKEKB 𝑒⁺⁢𝑒⁻ collider,more » we observe no evidence for a signal. We set upper limits on the product of the production cross section 𝜎⁡(𝑒⁺⁢𝑒⁻ → ℎ′⁡𝜒₁⁢𝜒₂), and the product of branching fractions ℬ⁡(𝜒₂ → 𝜒_1⁢𝑒⁺⁢𝑒⁻) ×ℬ⁡(ℎ′ → 𝑥⁺⁢𝑥⁻), where 𝑥⁺⁢𝑥⁻ indicates 𝜇⁺⁢𝜇⁻, 𝜋⁺⁢𝜋⁻, or 𝐾⁺⁢𝐾⁻, as functions of ℎ′ mass and lifetime at the level of 10⁻¹ fb. We set model-dependent upper limits on the dark Higgs mixing angle at the level of 10⁻⁵ and on the dark photon kinetic mixing parameter at the level of 10⁻³. This is the first search for dark Higgs bosons in association with inelastic dark matter.« less

We report the first observation of the two-body baryonic decays 𝐵⁺ → Σ_𝑐⁢(2455)⁺⁺$$\bar{Ξ}^-_c$$ and 𝐵⁰ → Σ_𝑐⁢(2455)^0⁢$$\bar{Ξ}^0_𝑐$$ with significances of 7.3⁢𝜎 and 6.2⁢𝜎, respectively, including statistical and systematic uncertainties. The branching fractions are measured to be ℬ⁡(𝐵⁺ → Σ_𝑐⁢(2455)⁺⁺$$\bar{Ξ}^-_c$$) = (5.74 ± 1.11 ± 0.4⁢2$$^{+2.47}_{−1.53}$$) ×10⁻⁴ and ℬ⁡(𝐵⁰ → Σ_𝑐⁢(2455)^0⁢$$\bar{Ξ}^0_𝑐$$) = (4.83 ± 1.12 ± 0.3⁢7$$^{+0.72}_{−0.60}$$) ×10⁻⁴. The first and second uncertainties are statistical and systematic, respectively, while the third ones arise from the absolute branching fractions of $$\bar{Ξ}^-_c$$ or $$\bar{Ξ}^0_𝑐$$ decays. The data samples used for this analysis have integrated luminosities of 711 fb⁻¹ and 365 fb⁻¹, and weremore » collected at the ϒ⁡(4⁢𝑆) resonance by the Belle and Belle II detectors operating at the KEKB and SuperKEKB asymmetric-energy 𝑒⁺⁢𝑒⁻ colliders, respectively.« less

We present the results of a search for the charged-lepton-flavor violating decays B⁰ → K^*0τ^±ℓ^∓, where ℓ^∓ is either an electron or a muon. The results are based on 365 fb⁻¹ and 711 fb⁻¹ datasets collected with the Belle II and Belle detectors, respectively. We use an exclusive hadronic B-tagging technique, and search for a signal decay in the system recoiling against a fully reconstructed B meson. We find no evidence for B⁰ → K*⁰τ^±ℓ^∓ decays and set upper limits on the branching fractions in the range of (2.9–6.4)×10⁻⁵ at 90% confidence level.

We measure the 𝐶⁢𝑃 asymmetry in 𝐷⁺ → 𝜋⁺⁢𝜋⁰ decays reconstructed in 𝑒⁺⁢𝑒⁻ collisions at the Belle II experiment using a dataset corresponding to an integrated luminosity of 428 fb⁻¹. A control sample of 𝐷⁺ → 𝜋⁺⁢𝐾$$^0_𝑆$$ decays is used to correct for detection and production asymmetries. The result 𝐴_𝐶⁢𝑃⁡(𝐷⁺ → 𝜋⁺⁢𝜋⁰) = (−1.8 ± 0.9 ± 0.1)%, where the first uncertainty is statistical and the second systematic, is the most precise determination to date. It agrees with the prediction of 𝐶⁢𝑃 symmetry from the standard model and with results of previous measurements.